This invention relates to a piston and wristpin assembly that permits the piston to have an axially symmetrical structure for use in high temperature, high pressure reciprocal engines. The piston and wristpin assembly is particularly useful for two-cycle diesel engines where thermal effects deform the symmetry of conventional pistons and where high thrust pressures of the piston at the connecting wristpin result in lubrication failures.
Piston and wristpin designs for conventional engines result in an asymmetric structure in the piston and a limited projection surface on the cylindrical wrist pin onto which all forces of the piston power stroke are transmitted. These two features of conventional design limit the permissible maximum temperatures and pressures in the combustion chamber. When excessive temperatures are encountered by a conventional piston, the piston expands in a nonuniform manner resulting in an overall asymmetry that increases friction and may result in piston slap.
Furthermore, in conventional combustion chamber designs the piston head or crown is subjected to the high temperatures of combustion and will thermally expand, particularly when the engine is running at higher speed and loads, such that acceptable tolerances at low speeds are insufficient to accommodate high speed, high load operations resulting in increased friction and excessive wear.
In conventional wristpin designs, the projected surface area of the cylindrical pin is approximately only 25-30% of the total cross sectional area of the piston. This limited effective surface area must bear the entire thrust forces of the piston during its power stroke. As precompression and combustion systems improve, a point is reached in which the effective surface area is no longer able to withstand the thrust forces and the thin lubrication film breaks down causing scoring and potential seizure of the bearing surface of the wrist pin. Even where this critical pressure per unit area is not reached, excessive wear occurs on a limited surface of the wrist pin since the reciprocal angular motion of the connecting rod results in virtually all wear being directed to a small top arcuate surface of the pin. This concentration of forces to a limited area also inhibits effective lubrication even under normal operating conditions.
The asymmetrical design of conventional cross pin pistons and the conventional designs of the connecting wristpin have inhibited the ability of engines to combust fuel at higher pressures and temperatures. This is a particular problem in two cycle engines where the piston has a minimal recovery from the high temperature power stroke before the next power stroke, unlike the four stroke cycle. Furthermore, when a two cycle system is adapted to diesel operation, with autocombustion of injected fuel, piston deformation and wrist pin overload become major considerations even under conditions of only moderate supercharging. The axially symmetric piston and the novel wrist pin structure of this invention resolve the problems created in conventional piston and wrist pin designs and allow engine systems to be created that substantially increase the operating pressures and temperatures without interfering deformation or expansion of the piston or failure in the wristpin lubrication.